This invention relates generally to a high intensity discharge (HID) lamp ballast and, more particularly, to an improved scheme for ignition of an HID ballast.
An HID lamp generally includes high pressure mercury, high pressure sodium, metal halide, high pressure metal vapor and low pressure sodium lamps. Each of these lamps is ignited through application of a high voltage pulse, nominally of several thousand volts, across the electrodes of the lamp. Following lamp ignition, the lamp enters a glow stage in which very little (i.e. a stream of) current initially flows between the electrodes of the lamp. Once the mercury, metal halides or salts serving as the plasma within the lamp has been sufficiently vaporized, the lamp enters an arc stage in which an arc rather than a stream of current continuously flows between the lamp electrodes.
The glow stage, which typically can last up to several milliseconds, includes both an unstable period and a stable period. During the unstable period (commonly referred to as "takeover"), which precedes the stable period and which begins immediately following lamp ignition, the lamp voltage drops from, for example, several thousand volts to several hundred volts. Following take over (which typically lasts for about 1-3 microseconds after lamp ignition) and for the remainder of the glow stage (i.e. the glow stage stable period), the lamp voltage alternates between several hundred volts and a much lower voltage level (e.g. about 20 volts for a metal halide type) prior to entering the arc stage.
A conventional HID ballast includes a down converter. The down converter serves as a current source for the ballast and must therefore meet the ever increasing current demands of the HID lamp during takeover and otherwise during the glow stage. The down converter increases its current output slowly and frequently is unable to meet the current demand. When the current demanded by the HID lamp during takeover or otherwise during the glow stage cannot be met, the stream of current between lamp electrodes will extinguish.
The lifetime of a HID lamp is decreased each time the lamp is ignited due to sputtering of the emissive electrode material. The sputtered, emissive electrode material eventually covers the inner lamp wall reducing lumen output and can result in flicker.
Accordingly, it is desirable to provide an HID ballast having improved ignition properties so as to minimize the need for re-ignition of the lamp during takeover and otherwise during the glow stage. In particular, the HID ballast should minimize the possibility of arc extinction due to insufficient current being available from the down converter during takeover and otherwise during the glow stage. The improved HID ballast should make it possible for the discharge lamp to more quickly enter the arc stage, increasing lamp life and maintaining for a longer period of use the nominally rated lumen output of the lamp.